Many types of crash sensors have been proposed and used for determining if a crash is severe enough to require the deployment of a passive restraint system such as an air bag or seatbelt tensioner.
Three types of sensors, in particular have been widely used to sense and initiate deployment of an air bag passive restraint system. These sensors include an air damped ball-in-tube sensor such as disclosed in Breed U.S. Pat. Nos. 3,974,350, 4,198,864, 4,284,863, 4,329,549 and 4,573,706, a spring mass sensor such as disclosed in Bell U.S. Pat. Nos. 4,116,132, 4,167,276 and an electronic sensor such as is part of the Mercedes air bag system. Each of these sensors has particular advantages and shortcomings which will be disclosed below.
The choice of the sensor technology to be used on a given vehicle depends on where the sensor is mounted. When a car is crashing only certain portions of the vehicle are crushing at the time that the sensors must trigger to initiate timely restraint deployment. A car, therefore, can be divided into two zones: the crush zone which has changed its velocity substantially relative to the remainder of the vehicle and the non-crush zone which is still travelling at close to the pre-crash velocity. To sense a crash properly in the crush zone the sensors must function as a velocity change indicator; that is, the sensor must trigger at approximately a constant velocity change regardless of the shape or duration of the crash pulse. The response characteristic for a sensor in the non-crush zone must be determined experimentally and generally has a form that for a high deceleration a small velocity change will trigger the sensor and for lower decelerations a larger velocity change is required.
Air damped ball-in-tube crash sensors are inherently velocity change indicators and are the only sensors which have found widespread use for mounting in the crush zone. Spring mass sensors inherently trigger at smaller velocity changes for high deceleration levels and high velocity changes for low deceleration levels and therefore have only found widespread applicability in the non-crush zone locations of the car. Electronic sensors could be designed to function in either manner and thus theoretically could be placed either in the crush zone or in the non-crush zone.
Each of these sensors has significant limitations. If spring mass sensors are placed in the crush zone either they will trigger on very short duration low velocity change crush pulses where a restraint system is not needed or they will not trigger on longer duration pulses where a restraint is needed, depending on the particular sensor design. In addition, since the motion of the mass in the spring mass system is undamped, it has been very difficult to get reliable contact closure on vigorous crash pulses where the mass bounces back and forth many times. To solve this contact problem, spring mass sensors are frequently placed slightly out of the crush zone for frontal barrier impacts. In this case, however, they sometimes become in the crush zone for angle car to car impacts for example, and are prone to both triggering when a restraint is not desired and the contact problems discussed above.
Electronic crash sensors have so far only been used in protected passenger compartment non-crush zone locations. Most electronic sensors have environmental limitations which are exceeded by crush zone locations which are frequently near the engine or radiator. Newer electronic technologies, however, have overcome these environmental limitations and consideration can now be given to crush zone mounted electronic sensors.
Ball-in-tube sensors can be designed to operate either in the crush zone or in the passenger compartment. However, their primary advantages lay in the crush zone. When used in the non-crush zone they trigger slightly faster than a spring mass sensor and slightly slower than an electronic sensor. Ball-in-tube sensors suffer from several significant technical problems. The sensor triggers properly only when responding to longitudinal decelerations. When cross axis accelerations, such as in the vertical and lateral directions are present the ball can begin whirling or orbiting around inside the cylinder resulting in a significant change in the response of the sensor. In one case, for example, a crash sensor would trigger on a 10 mile per hour velocity change in the absence of cross axis vibrations but require as much as a 13 MPH velocity change when the cross axis vibrations are comparable in magnitude to those frequently experienced in the crush zone of a vehicle. One automobile manufacturer had the requirement that an air bag not deploy at 9 MPH or below but must deploy at a 12 MPH or above for impacts into a barrier. The ball-in-tube sensor, due to cross axis effects, was not capable of meeting this requirement and thus the requirement was modified to an 8 MPH no trigger and a 14 MPH all trigger requirement. Thus the ability of the restraint system to protect occupants in marginal crashes has been severely compromised.
The ball-in-tube sensor depends upon the viscous flow of air between the ball and the tube to determine the characteristics of the sensor. The viscosity of air is a function of temperature and although materials are selected for the ball and the tube to compensate for the viscosity change, this compensation is not complete and thus the characteristics of the ball-in-tube sensor will inherently vary with temperature. To achieve the best temperature compensation requires control of the composition of the alloys used for the ball and tube which are considerably beyond normal commercial practice.
In addition, the biasing force which is used to hold the ball at its home position when a vehicle is not in a crash is provided by a ceramic magnet for the ball-in-tube crush zone sensor. This biasing force has a significant effect on the threshold triggering level for long duration pulses such as impacts into snow banks or crash attenuators which frequently surround dangerous objects along the highways. Due to the temperature effects on the magnet, this biasing force changes by about 40% over the desired temperature operating range of the occupant restraint system.
To function properly, a crush zone sensor of any design must be in the crush zone. Any crush zone sensor which is based on a mass sensing deceleration has a potential of triggering very late if it is not in the crush zone for a particular crash. This is particularly a problem with ball-in-tube sensors which have a very low bias. One example of this involved a stiff vehicle in a low speed barrier impact where the sensor was not sufficiently forward in the car and thus not in the crush zone. The sensor triggered when the entire velocity change of the car reached 10 MPH at which time the occupant was leaning against the air bag. An occupant who is severely out of position and close to the air bag when it deploys can be seriously injured by the deploying air bag.
A further shortcoming of all mechanical sensors is that in very vigorous crashes the sensing mass can bounce at the end of its travel, resulting in very short contact closure which may be insufficient to provide enough energy to initiate restraint deployment.